A shallow, moderately sized magnitude-5.6 crustal earthquake rattled West Java, Indonesia, this week, causing shaking and loss.
By Anne Sirait, Ph.D., Department of Geophysics, University of Indonesia and a member of Seismology Division of Himpunan Ahli Geofisika Indonesia (HAGI).
Citation: Sirait, A., 2022, A shallow crustal earthquake impacts West Java, Temblor, http://doi.org/10.32858/temblor.284
Artikel ini juga tersedia dalam bahasa Indonesia.
Time: Monday, Nov. 21, 2022, around 1 p.m.
Place: Depok, West Java, Indonesia.
My Mathematical Physics class was preparing for a quiz. Ten minutes into the quiz, as I am watching my students try to answer the questions, the desks, screen projector and curtain start to shake.
It’s an earthquake.
Fortunately, we remember all our training for when an earthquake happens. We get under the table and cover our heads until the shakes stop. Since we are at the fourth floor, we need to go down a stairwell to exit the building. We manage to keep ourselves calm and orderly. We wait for about half an hour before we go back to the classroom. The quiz then continues, since we were a safe distance from the epicenter and did not experience any building collapse.
The earthquake that shook the class was a magnitude-5.6 event, with the epicenter at Sukalarang Regency, West Java, Indonesia. This earthquake shook much of the western part of the island. The earthquake struck at 1:21 p.m. local time. According to the Badan Meteorologi, Klimatologi, dan Geofisika (BMKG), the national seismic agency in Indonesia, the earthquake was located onshore, 11 kilometers deep. Because it occurred onshore, no tsunami could possibly happen. Within an hour of the mainshock, 25 aftershocks were recorded, ranging from magnitude 1.8 to magnitude 4.0.
Shaking varied across West Java. Near the epicenter, intense shaking of V on the Modified Mercalli Intensity Scale (MMI scale) was felt by all the people located in the city of Cianjur (about 40 kilometers, or about 25 miles, from the epicenter), according to BMKG. But people more than 100 kilometers (about 62 miles) from the epicenter also felt the quake, including my students and me in Depok, where we felt shaking at about level II on the scale. Other places that experienced shaking include Garut, and Sukabumi (IV MMI scale), Rangkasbitung and Bogor (III MMI scale), and the capital city of Jakarta (II to IV MMI scale, likely variable because people in taller buildings can experience more shaking than those on the ground).
Heavy damage was reported at Cianjur, a town near the epicenter, and surrounding areas. Houses, schools, public infrastructures and facilities experienced heavy damage and/or collapse. Several landslides occurred due to the shaking. The worst hit area is Cugenang sub-district, where one village was buried by a landslide. For the last 24 hours, it has been reported that there were 268 casualties, 1,083 people injured, and 151 people still missing. More than 22,000 houses were damaged, causing tens of thousands of people to evacuate from their homes. The casualties are due to collapse of houses and landslides. Much of the houses and infrastructure have not been built to be earthquake resistant. Most of the houses that were made of bricks experienced heavy damage, while houses made of wood were not broken.
The Cimandiri Fault
Analysis by BMKG suggests this earthquake might be related to activity on the Cimandiri Fault, one of the major faults on West Java. Based on the seismic data analysis, the source mechanism of this earthquake is a strike-slip, which is consistent with the mechanism of the Cimandiri Fault, a left-lateral strike-slip fault.
Based on the Peta Sumber dan Bahaya Gempa Indonesia by Pusat Gempa Nasional (PUSGEN, 2017), the Cimandiri consists of three segments: Cimandiri segment in the south, Nyalindung-Cibeber segment in the middle, and Rajamandala segment in the north. Monday’s earthquake epicenter was located closer to the Rajamandala segment of the Cimandiri Fault. But, this analysis is preliminary; more study is needed to ascertain the role of any segment of the Cimandiri Fault.
The Cimandiri Fault moves at a rate of 4 millimeters per year (Safitri, 2016) and has a potential to generate a magnitude-7.2 event, about 100 times more energy than the one that happened on Monday. The fault has hosted several historical earthquakes: a quake with VII shaking intensity in 1900 (Visser, 1922), a magnitude-5.5 earthquake in 1982, and magnitude-5.4 and magnitude-5.1 events in 2000 (Supartoyo et al., 2008). Between 2009 and 2015, there were 10 earthquakes identified as occurring along the Cimandiri Fault (PUSGEN, 2017).
Looking at the periodicity of earthquake occurrences on the Cimandiri Fault — about 20 years in time (1982, 2000, and 2022) — we need to be prepared for another earthquake in the future, regardless of whether this one was on the Cimandiri Fault or not.
What to expect?
The Cimandiri Fault cuts through a highly densely populated area, including Pelabuhan Ratu, Sukabumi, Cianjur and Padalarang. Looking at the impact of Monday’s earthquake and its impact on the population, we are reminded of the need for hazard education and mitigation.
Education on natural hazards is needed to make people aware of the existence of active faults and areas that are vulnerable to the shaking that may lead to landslides in their surroundings. The damage to houses and infrastructure due to shaking that led to causalities is a reminder to be more diligent in implementing an appropriate building code. Houses and infrastructure must be built to be earthquake resistant. The building code exists, but is hardly implemented. We need to encourage all to implement the code to have safer buildings, and therefore, fewer losses.
Pusat Studi Gempa Nasional (PUSGEN). (2017). Peta sumber dan bahaya gempa Indonesia tahun 2017, Tim Pusat Studi Gempa Nasional, Pusat Litbang Perumahan dan Pemukiman, Balitbang PUPR, Bandung, Indonesia, ISBN: 978-602-5489-01-3 (in Bahasa Indonesia).
Safitri, A & Meilano, I & Gunawan, Endra & Abidin, Hasanuddin Z. & Efendi, J & Kriswati, Estu. (2018). Strain Variation along Cimandiri Fault, West Java Based on Continuous and Campaign GPS Observation From 2006-2016. IOP Conference Series: Earth and Environmental Science. 132. 012027. 10.1088/1755-1315/132/1/012027.
Supartoyo, E. T. Putranto, And Djadja (2005), Active Faults and Destructive Earthquake
Epicenter Distribution Map of Indonesia.
Visser, S. W. (1922), Inland And Submarine Epicentra Of Sumatra And Java Earthquakes, Javasche Boekhandel En Drukkerij.